SEROLOGY MANUFACTURE AND USE OF SERA AND VACCINES 263 



From nine to twelve liter of blood are taken from the horse at one time and 

 the bleeding is repeated four or five times at intervals of about six months. 

 The punctured wound is closed by keeping an artery forceps in position 

 for a short time. 



E. Securing the Serum. The blood tubes are set aside until the clot 

 has formed and settled to the bottom. The clear serum is siphoned off 

 into a large flask, 0.25 per cent, of trikresol is added as a preservative and 



FIG. 63. Bleeding the horse after a maximum amount ot the antitoxin of diph- 

 theria has been developed in the blood. The animals pay but little attention to the 

 operation. 



to kill any germs that might be accidentally present, and then filtered 

 through several thicknesses of filter paper, under pressure (suction). The 

 perfectly clear, sterile and germ-free serum constitutes the antitoxin of 

 diphtheria and is ready for use as soon as it is standardized and put into 

 suitable containers. 



F. Standardizing the Antitoxin of Diphtheria. Since the antitoxic va- 

 lence of horse serum as above described varies somewhat, it is necessary to 

 determine the quantitative value in order that physicians may know what 

 amounts to administer in the treatment of diphtheria. The standard 

 unit of strength now adopted by all civilized countries is the so-called 



